The invention relates to a luminescent quartz glass activated by a rare earth metal and comprising as a glass matrix oxide: mainly SiO.sub.2 ; and furthermore 0 to 0.15 mol of MO per mol of SiO.sub.2, in which MO represents at least one of the oxides Al.sub.2 O.sub.3, B.sub.2 O.sub.3, P.sub.2 O.sub.5, ZrO.sub.2, Sc.sub.2 O.sub.3, Y.sub.2 O.sub.3, La.sub.2 O.sub.3, Gd.sub.2 O.sub.3, Lu.sub.2 O.sub.3, the alkali metal oxides and the alkaline earth metal oxides. The invention also relates to a method of preparing such a luminescent quartz glass and to a luminescent screen provided with such a luminescent quartz glass.
Luminescent quartz glasses have been known for a comparatively long time. U.S. Pat. Nos. 3,527,711, 3,634,711, 3,855,144 and 3,935,119, for example, describe such glasses whose SiO.sub.2 content is very high (generally at least 99% by weight) and in which preferably rare earth metals are used as activators. These rare earth metal activators are used in very low concentrations with a maximum concentration of 5000 ppm (rare earth metal atoms relative to Si atoms) being stated. Also if a plurality of rare earth metals is used as an activator, the concentration of all activators combined is not to exceed this maximum value and preferably the limit of 2000 ppm is not exceeded. Expressed in mol.% of the (trivalent) rare earth metal oxide relative to SiO.sub.2, the said limits of 5000 and 2000 ppm are 0.25 and 0.1 mol%, respectively. In the above cited patents it is stated that less transparent products are obtained when the given maximum values of the activator concentration are exceeded. Less transparent and coloured luminescent materials have the drawback of a too low luminous flux so that they are not usable in practice.
U.S. Pat. No. 3,459,673 describes a method of preparing a quartz glass activated by rare earth metals in which the starting material is a hydrolysable silicon compound and an aqueous solution of a salt of a rare earth metal soluble in water. Also in this publication very low activator concentrations are stated (in the examples, concentrations of the order of several hundred ppm are used).
A luminescent quartz glass of the type described in the opening paragraph is known from Netherlands patent application No. 6605957. In this application a terbium-activated glass is described whose glass matrix, that is to say, the glass composition without an activator oxide, mainly consists of SiO.sub.2 but also comprises up to approximately 0.20 mol per mol of SiO.sub.2 of other oxides mentioned by name. According to this patent application the addition of these other oxides renders a higher terbium concentration possible. The maximum concentration is, however, only 0.5 mol% of Tb.sub.2 O.sub.3 per mol of SiO.sub.2, that is to say, only a factor of 2 higher than the maximum concentration in the aforementioned United States patents.
A great drawback of the known luminescent quartz glasses is that they have a low efficiency compared with crystalline luminescent materials. Particularly the quantum efficiency upon excitation by ultraviolet radiation is often found to be a factor of 5 to 10 smaller than the quantum efficiencies which can be achieved with efficient crystalline materials.
The object of the invention is to provide luminescent quartz glasses having high conversion efficiencies and notably high quantum efficiencies upon excitation by ultraviolet radiation.